It is to be understood that the term “wavelength” as used herein in reference to the physical distance between successive equal-phase locations of a radio-frequency electromagnetic signal (the sense used throughout for the term of art “RF”), has a first definition in free space, wherein signals axiomatically propagate at the speed of light (C), a second definition in air, which is slightly shorter, and additional definitions on single conductors in free space and elsewhere, in transmission lines with air or another dielectric material present between two or more conductors in whole or in part, and in waveguides, delay lines, and other environments. For example, wavelength of a gigahertz-range signal in a 50 ohm coaxial line sized for transmission of multi-watt signals, made from high-conductivity copper, and having an air dielectric with “beads” (spacer disks) of solid polytetrafluoroethylene providing about 1% fill should be calculated using a dielectric constant ∈ of about 1.03 for the air-filled portion and about 2.0 for the PTFE portion, summing to an equivalent of around 1.05. Thus, the physical length measured in wavelengths of such a mostly-air-filled coaxial line may be on the order of 80% as long as a radiated electromagnetic signal of the same frequency in free space. For simplicity, this disclosure generally assigns a dimension for “wavelength” that is adjusted according to the propagation environment except where the effects of differing propagation rates affect apparatus operation sufficiently to introduce ambiguity.
There has recently been an industry focus on digital streaming of content to mobile, portable, and handheld receivers through terrestrial broadcast systems. This type of broadcasting is being developed for implementation in licensed UHF frequency bands such as 0.7 GHz to 1 GHz (upper L-Band: TV channel 52 and above; mobile radio) and 1 GHz to 2 GHz (lower S-band).
At L-Band frequencies, the preferred method of transmission is vertical polarization. There are at present two styles of vertically polarized antennas that are readily available for commercial use in transmission at these microwave frequencies, namely panel and whip antennas. Panel antennas are intrinsically directional in nature and are typically used to cover sectors of space. Whip antennas are nominally omnidirectional and are used preferentially in applications requiring substantially equal radiation in all azimuths.
Traditional whip antennas for UHF are limited in power handling, and are further limited in elevation radiation pattern flexibility. These antennas are, in many embodiments, constructed by interleaving collinear dipoles in an array, center feeding the array, and establishing a phase difference between the upper and lower halves of the array to provide beam tilt. A whip antenna formed from an interleaved collinear array is shown in FIG. 1.
The design shown in FIG. 1 uses dipoles that are each approximately one-half wavelength long (for a nominal frequency) and are connected to one another in parallel. The inner conductor of the first dipole is electrically connected to the outer conductor of the next dipole. This inner-to-outer interleaving at half-wave intervals compensates the radiated phase of all the dipoles so they are in phase at one-half wave (180 degree) spacing. This pattern continues until the dipole strings are terminated in shorts at the furthest positions from the center input feed.
The center feed arrangement shown in FIG. 1 inherently limits the amount of beam tilt that can be achieved, with no more than about 2 degrees of tilt possible before the elevation pattern develops a split beam and the gain is severely degraded. As is typical in parallel arrays, the dipoles add in parallel to determine the nominal gain.
One consideration in building long arrays of elements is that as array length increases, the connection of additional elements results in increased input impedance mismatch. This in turn increases the transformation ratio needed in an input feed to bring the impedance back to, for example, 50 ohms. Increasing the transformation ratio reduces input impedance bandwidth and antenna power handling capability.
Another consideration in whips using strings of dipoles is mechanical stability. The dipoles in known arrangements are mounted inside a relatively thick fiberglass tube to provide necessary mechanical support. This radome can introduce attenuation.
To summarize, the shortcomings of a vertically polarized collinear dipole antenna include:
Limited beam tilt can be realized.
Increased input loading with additional dipoles constrains input transformer performance for both power and bandwidth.
Structural support is provided largely by the radome.
Panel antennas involve tradeoffs different from those for whips. Considerations may include requirements to provide extensive systems of power dividers and feed lines where multiple panels must receive individual and carefully phased inputs, a panel or an array of panels pointing in each direction (typically four quadrants for omnidirectional capability, with gain dependent on array size), use of a tower with multiple discrete units mounted thereon, and management of significant wind loading. While very high power capability and precise beam control can be supported, high efficiency at moderate power may be uneconomical.
Accordingly, it is desirable to provide an apparatus and method for a vertically polarized traveling wave antenna that permits simplicity in its mechanical construction, minimal design adaptation to vary beam tilt and null fill, matched input impedance substantially independent of the number of elements, excellent azimuth pattern circularity, and moderate power capability.